1. Field of the Invention
The present invention relates to a thermal and current sensing switch capable of breaking an electric circuit in response to an overcurrent or a short-circuit current to protect overcurrent accidents and breaking the electric circuit to protect excessive temperature rise upon the detection of rise in the ambient temperature.
2. Prior Art
It is generally known to employ a current fuse to break an electric circuit when an overcurrent is supplied to the electric circuit, and to employ a thermal fuse or a thermostat which breaks an electric circuit upon the detection of an abnormal rise in the ambient temperature to protect the excessive rise in the temperature of the electric apparatus. Generally, the current fuse, and the thermal fuse or the thermostat are provided individually in the electric apparatus.
Efforts have been made to break an electric circuit by the action of a thermal fuse when an overcurrent flows through the electric circuit as well as responding to a rise in the ambient temperature.
Furthermore, efforts have been made to protect both excessive rise in the temperature of an electric apparatus and overcurrent accidents in the electric circuit by the action of a circuit protecting device comprising a series arrangement or an integral combination of a thermal sensing fuse element and an overcurrent sensing fuse element.
With the thermal fuse or the like for protecting excessive temperature rise, it is most important that the thermal fuse responds to ambient temperature at a high accuracy, i.e. at least within the range of several degrees centigrade for the predetermined temperature. However, heat generated in the electric apparatus by a current flowing through the electric circuit adversely affects the temperature accuracy of the thermal fuse.
With a current fuse or the like for protecting overcurrent, relation of response times to several values of overcurrents resulting from overloading, for example, a current of 135% or 200% of the rated current of the circuit, namely, a current-time characteristic, is an important factor. Furthermore, ability of safely and surely breaking the associated electric circuit in response to a large current, such as a short-circuit current, namely, breaking capacity, is another important factor of the current fuse. Generally, the thermal sensing device is placed in a place of a temperature higher than a room temperature when the associated electric circuit is in the normal operating state. Therefore, the overcurrent sensing ability of the current fuse must not be subject to the influence of the ambient temperature.
Since a thermal fuse is designed so that the influence of Joule heat is eliminated almost completely, when a thermal fuse is used to brake an electric circuit by the Joule heat, the thermal fuse does not respond to an overcurrent not much higher than the rated current and it is unable to break the associated electric circuit safely and surely when a very high overcurrent, particularly, a short-circuit current, flows through the electric circuit because its contacts weld or its fuse element explodes. Accordingly, such a thermal fuse is effective only with an overcurrent in a narrow range.
The thermal sensing fuse element and the overcurrent sensing fuse element of the foregoing circuit protecting device are connected in an electrically conductive manner, and hence the thermal sensing fuse element is subject to the influence of Joule heat. When the overcurrent sensing fuse element is formed of an alloy such as tin, lead or bismuth having a comparatively low melting point to suppress temperature rise attributable to Joule heat evolved by a normal operating current, the fuse element fuses at a low temperature when an overcurrent flows therethrough, the current-time characteristic of the current fuse is affected greatly by slight rise in the ambient temperature, and it happens that the current fuse is deteriorated or is caused to fuse by a current lower than the rated current. Furthermore, the breaking speed of a current sensing device employing a fuse element having a comparatively low melting point under an overcurrent is low and the breaking capacity of the same for a large current is small, because capacity of the fusible element is large.
When a fuse element having a comparatively high melting point, such as a silver, copper, nickel or tungsten fuse element, is employed to give priority to the current characteristic of the circuit protecting device, increased heat is generated at the normal operating current to enhance the influence of heat on the thermal sensing element remarkably deteriorating the thermal sensing accuracy.
The response temperature of some thermal sensing element is determined taking into consideration both the influence of heat generated by the current flows through the thermal sensing element itself and that of heat generated by the current flows through the associated current sensing element. However, it is difficult to stabilize the temperature sensing accuracy of such a thermal sensing element under a high operating current because the temperature sensing accuracy is affected significantly by the assembling accuracy, the quality of the material and small dimensional errors, and hence only thermal sensing elements of such a type having a small current capacity are feasible. When the response temperature is determined taking into consideration temperature rise resulting from heat evolved by a current in addition to the ambient temperature, a constant current must be supplied to the associated circuit at all times, which, however, is difficult to be met by general electric equipments.
Basically, the current fuse must respond sharply to Joule heat evolved by a current and is not significantly susceptible to the ambient temperature, while the thermal fuse must respond sharply to the ambient temperature and must not be susceptible to Joule heat evolved by a current. Such requirements apply also to heat sensing devices other than thermal fuses, such as thermostats.
It has been difficult for the prior art to provide a device meeting both the basic characteristics contrary to each other; giving priority to one of the basic characteristics unavoidably entails the degradation of the other.
Accordingly, it has been necessary to provide an electric circuit individually with a protective device for protecting the electric circuit from overcurrent and a protective device for protecting excessive temperature rise to ensure the protection of burning accidents in the electric apparatus.